Digital lighting technologies, i.e. illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g. red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626, the disclosures of which are specifically incorporated herein by reference.
As is known, the lifetime of an LED is related to the junction temperature; the greater the junction temperature, the shorter the lifetime of the LED. LED lifetime requirements based on the junction temperature of the LEDs are often specified at the maximum ambient temperature rating of the product. Illustratively, the lifetime requirement is fifty thousand hours of operation at 50° C., with the understanding that the higher the ambient temperature, the higher junction temperature of the LED, leading to shorter lifetime. Often, LEDs designed to this standard are driven at a particular drive current to attain an output power. In order to meet the lifetime requirements, the power output to the LEDs in known LED-based lighting fixtures is set at the same level regardless of the ambient temperature. For example, the power output level is selected for the maximum ambient temperature and junction temperature to meet the lifetime specification. Naturally, at a lower ambient temperature and junction temperature, the drive current to the LEDs is lower for the output power selected for maximum ambient and lifetime criteria. Illustratively, at ambient temperatures in the range of 25° C. to 30° C., at the selected output level, the junction temperature of the LEDs, the lifetime is increased over that of the requirements, but is realized at the cost of reduced output power. Accordingly, because of the design criteria for LED lifetime are based on comparatively high ambient temperatures (e.g., 50° C.), known LED-based lighting fixtures operating at typical ambient temperatures (e.g., 25° C. to 30° C.), are not driven with the maximum current possible for the lifetime requirements.
Thus, there is a need in the art to provide LED-based lighting fixtures that have a greater power output over typical ambient temperature ranges while complying with lifetime specifications for higher ambient temperatures.